pH Electrode glass compositions

ABSTRACT

A glass composition for use in pH-responsive glass electrodes, and a glass electrode having a pH-responsive membrane of such glass, which composition consists essentially of (in mole percent on the oxide basis): 
     a. from about 30 to about 37 mole percent Li 2  O; 
     b. from 0 to about 4 mole percent of at least one oxide selected from the group consisting of Cs 2  O and Rb 2  O; 
     c. from about 2 to about 12 mole percent of at least one oxide selected from the group consisting of La 2  O 3  and Pr 2  O 3  ; 
     d. from about 2 to about 10 mole percent of at least one oxide selected from the group consisting of Ta 2  O 5  and Nb 2  O 5  ; 
     e. from 0 to about 4 mole percent UO 2  ; and 
     f. the balance SiO 2  ; 
     with the proviso that there is present in the composition at least about 2 mole percent of at least one oxide selected from the group consisting of Pr 2  O 3  and Nb 2  O 5 .

BACKGROUND OF THE INVENTION

This invention relates to pH electrode glass compositions. Moreparticularly, this invention relates to pH electrode glass compositionshaving very low bulk resistivities and very low surface resistivitiesafter aging in aqueous solutions.

Glass electrodes sensitive to the hydrogen ion activity, or pH, of asolution have been known for some time. Since their discovery in 1906and the various modifications and improvements in glass compositionswhich followed, such electrodes have come to play an important role inboth research and industry.

Desirable properties in a pH glass electrode, which largely arefunctions of the glass composition and configuration, include thefollowing:

a. Low volume or bulk resistivity. Potentiometric error due to offsetcurrent and finite impedance at the input of the electrode potentialmeasuring means, e.g., a pH meter, is directly proportional to the totalresistance of the pH electrode. Consequently, low bulk resistivity in apH glass allows fabrication of smaller or thicker membranes for suchpurposes as smaller size, increased strength, and attainment of requiredgeometries, while retaining the good performance of the more commonlarger or thinner membranes.

b. Low surface resistivity. Decreased surface resistance reduces thetendency of pH electrodes to polarize. The term "polarize" is commonlyused to describe the long-term disturbance of pH electrode potentialoften induced by a brief current flow through the sensing glassmembrane. Because recovery from such disturbances typically is slow, fora period of time after a polarizing disturbance the pH measurement showsa slowly decaying error. The practical advantages of decreased surfaceresistance include enhanced speed of response, especially with highcapacitance cables, faster decay of transient potentials induced byexternal electrostatic coupling, and faster stabilization afterconnection to the pH meter and after changing bathing solutions. Theseadvantages are particularly important at low temperatures and withnoncombination electrodes which do not have a low-impedance referencejunction in the vicinity of the pH electrode to act as a sink for straycurrents.

c. Near-theoretical slope (volts/pH unit). Maintenance ofnear-theoretical slope over the entire pH range of 0-14 simplifiescalibration and allows pH measurements over the maximum range usuallyencountered.

d. Low sodium error. Low sodium error improves the accuracy of pHmeasurements in strongly alkaline solutions containing sodium ions.

e. Low asymmetry potential. Asymmetry refers to the potential differenceacross a membrane when the inner and outer surfaces are bathed withidentical solutions. A low asymmetry potential generally is associatedwith enhanced stability and uniformly in the potentiometriccharacteristics of manufactured electrodes.

f. High chemical durability. The pH glass should have sufficientchemical durability to allow a long life in strongly acidic or alkalinesolutions.

In general, it is not possible to obtain a single pH glass in which allof the desirable properties have been optimized. There usually is adegree of compromise associated with any pH glass electrode, with theend use or uses dictating which properties are of the greatestimportance. For example, the more recent pH glasses emphasize suchproperties as low sodium error, low bulk resistivity, improvedworkability, enhanced durability, and the absence of devitrification orphase separation. The most significant of these more recent pH glassesare described below.

U.S. Pat. No. 3,372,104 broadly discloses pH electrode glasscompositions which are lithia-silicate glasses provided from a pre-meltcomposition or mixture according to the following formula, expressed inranges of mole percentages on the oxide basis: (1) about 27-29 molepercent Li₂ O, (2) about 2-4 mole percent of at least one materialselected from the group consisting of Cs₂ O and Rb₂ O, (3) about 4-7mole percent of at least one rare earth metal oxide, (4) about 1-3 molepercent UO₂ and/or about 1-3 mole percent Ta₂ O₅, and (5) the balanceSiO₂ which typically is about 58-63 mole percent. The only rare earthmetal oxide actually used was La₂ O₃.

Glasses similar to the above are claimed in U.S. Pat. No. 3,410,777 andhave a composition consisting essentially of about 27-29 mole percentLi₂ O, about 2-4 mole percent of at least one material selected from thegroup consisting of Cs₂ O and Rb₂ O, about 4-7 mole percent of at leastone rare earth metal oxide, about 1-3 mole percent UO₂, and the balanceSiO₂. Although the only rare earth metal oxide actually used was La₂ O₃,the rare earth metal oxide can be selected from the group consisting ofLa₂ O₃ and Pr₂ O₃.

Finally, U.S. Pat. No. 4,028,196 discloses pH electrode glasscompositions consisting essentially of 30-40 mole percent Li₂ O, 50-60mole percent SiO₂, 2-8 mole percent La₂ O₃, 2-8 mole percent Ta₂ O₅, and0-3 mole percent Cs₂ O, wherein the sum of the mole percentages of Li₂ Oand Ta₂ O₅ is equal to or greater than 34.

Glass compositions such as those described above have provensatisfactory for the construction of pH glass electrodes having ageneral utility. For miniature, rugged, or flat-membrane pH glasselectrodes, however, there still is a need for glass compositions havingvery low bulk and surface resistivities and, as a consequence, a reducedtendency to polarize.

SUMMARY OF THE INVENTION

It therefore is an object of the present invention to provide apH-sensitive glass composition having very low bulk resistivity and verylow surface resistivity after aging in aqueous solutions.

It also is an object of the present invention to provide a glasselectrode having a pH-responsive membrane of glass having very low bulkresistivity and very low surface resistivity after aging in aqueoussolutions.

These and other objects will be apparent to those having ordinary skillin the art from a consideration of the specification and claims whichfollow.

It has been discovered that glasses containing certain rare earth metaloxides, notably praesodymium oxide, and certain group 5b metal oxidesexhibit properties that are particularly desirable in certain pHelectrode applications, i.e., pH electrodes having very small or thicksensing membranes. These properties include very low surface and bulkelectrical resistivities, good workability, and fast and accurateelectrode response to changes in pH. Such properties, of course, aredesirable for traditional pH electrode applications as well. Thus, theglasses of this invention have been found to be particularlyadvantageous for use in miniature, rugged, and flat-membrane electrodes,but are not limited to such uses.

Accordingly, the present invention provides a glass composition for usein pH-responsive glass electrodes, and a glass electrode having apH-responsive membrane of such glass, which composition consistsessentially of (in mole percent on the oxide basis):

a. from about 30 to about 37 mole percent Li₂ O;

b. from 0 to about 4 mole-percent of at least one oxide selected fromthe group consisting of Cs₂ O and Rb₂ O;

c. from about 2 to about 12 mole percent of at least one oxide selectedfrom the group consisting of La₂ O₃ and Pr₂ O₃ ;

d. from about 2 to about 10 mole percent of at least one oxide selectedfrom the group consisting of Ta₂ O₅ and Nb₂ O₅ ;

e. from 0 to about 4 mole percent UO₂ ; and

f. the balance SiO₂ ;

with the proviso that there is present in the composition at least about2 mole percent of at least one oxide selected from the group consistingof Pr₂ O₃ and Nb₂ O₅.

DETAILED DESCRIPTION OF THE INVENTION

The phenomena of surface resistance and polarization can be explained,in simplified terms, as follows. The total electrical resistance of ahydrated or aged pH glass membrane is the sum of two components, aninterior or bulk resistance and a surface resistance. In the interior ofthe glass, current is carried by highly mobile alkali metal ions(generally lithium ions in modern compositions and in the compositionsof the present invention) which move relative to fixed siloxy siteshaving a negative charge. Near the glass surface, however, current iscarried by hydrogen ions rather than by lithium ions. When a pH glass isimmersed in an aqueous solution, lithium ions progressively diffuse outof the glass surface and are replaced by hydrogen ions which have a muchhigher affinity for the fixed negative sites but much lower site-to-sitemobility. As a consequence of the lower hydrogen ion mobility, the bulkresistivity of the interior portions of the ion-exchanged glass surfaceexceeds the resistivity of the native, interior glass by a factor of atleast a thousand. The outer portions of the ion-exchanged surfaces,however, are hydrolyzed into much more permeable silica-gel networks oflow resistivity. Thus, during exposure to an aqueous environment, a verythin barrier layer of high resistivity glass progressively develops neareach surface at the interface between the more conductive bulk glass andthe silica-gel phases. For a more detailed discussion of the propertiesof hydrated pH glass surfaces, see, e.g., R. P. Buck, J. Electroanal.Chem., 18, 363 (1968), and A. Wikby, Physics and Chemistry of Glasses,15, 37 (1974).

The electrical properties of the hydrated pH glass membrane can beapproximated by the following idealized circuit model. The resistance ofthe bulk glass portion of the membrane can be represented by a fixedresistor, R_(b). The high-resistance barrier layers separating the bulkglass and the surface gel layers are quite thin and, therefore, areelectrically analogous to capacitors with a "leaky" (finite resistance)dielectric. Thus, the simplest circuit model which approximates theelectrical behavior of a hydrated pH glass membrane consists of aresistor R_(b) which is connected in series with a combination of aresistor R_(s) and a capacitor C_(s) connected in parallel, in which 1/2R_(s) and 2 C_(s) represent the resistance and capacitancecontributions, respectively, of each surface.

The above idealized electrical model correctly predicts a time-variantshift in potential when a direct current pulse is passed through a pHglass membrane. The graphical representation of a direct current pulseof fixed magnitude and duration is, of course, rectangular in waveform.When such a current pulse is passed through a new or hydrogenfluoride-etched pH glass membrane which has no surface resistance, theinduced voltage waveform also is rectangular and has a magnitude whichis proportional to the bulk resistance of the membrane. In an aged,i.e., hydrated, membrane having surface resistance layers, however, suchsurface layers result in an additional, time-varying component in theinduced voltage. Thus, the induced voltage rises abruptly to a magnitudewhich is proportional to the membrane glass bulk resistance. But theinduced voltage then continues to rise nonlinearly and more slowly withtime by an additional magnitude which is proportional to the sum of theresistances of the two surface layers. When the direct current pulse isterminated, the induced voltage drops abruptly to a magnitude which isproportional to the sum of the resistances of the surface layers. Thisresidual induced voltage then continues to drop nonlinearly with time,eventually returning to zero.

This nonlinear, delayed reduction of membrane potential is thephenomenon commonly referred to as polarization, which phenomenon is ofspecial significance. As a consequence of surface resistance, a briefelectrical disturbance passing current through a pH glass membrane has along-term residual effect on the measured electrode potential.

Electrical disturbances commonly are encountered during normal pHelectrode usage. For example, pH membranes routinely are subjected topolarizing currents as a consequence of direct charge transfer duringhandling, remote electrostatic coupling with charged worker apparel, andcharging of electrode cable capacitance to a new potential upon a changein pH. The adverse consequences of polarization susceptibility includegreater measurement error, slower response, and enhanced noise.

The electrical resistance properties of a glass membrane are readilymeasured by noting the time-variant shift in electrical potential acrossthe membrane which occurs in response to the application of aconstant-current pulse. The bulk resistivity of the electrode glass isgiven by the expression, ##EQU1## where A is the membrane area, l is themembrane thickness, and ΔV_(b) is the instantaneous change in electrodepotential upon application of current i. Similarly, the surfaceresistivity of the electrode membrane can be expressed as ##EQU2## whereΔVs is the change in electrode voltage during a fixed time intervalafter application of the current i.

As already indicated, the present invention broadly provides a glasscomposition for use in pH-responsive glass electrodes consistingessentially of (in mole percent on the oxide basis):

a. from about 30 to about 37 mole percent Li₂ O;

b. from 0 to about 4 mole percent of at least one oxide selected fromthe group consisting of Cs₂ O and Rb₂ O;

c. from about 2 to about 12 mole percent of at least one oxide selectedfrom the group consisting of La₂ O₃ and Pr₂ O₃ ;

d. from about 2 to about 10 mole percent of at least one oxide selectedfrom the group consisting of Ta₂ O₅ and Nb₂ O₅ ;

e. from 0 to about 4 mole percent UO₂ ; and

f. the balance SiO₂ ;

with the proviso that there is present in the composition at least about2 mole percent of at least one oxide selected from the group consistingof Pr₂ O₃ and Nb₂ O₅.

In a preferred embodiment, such glass composition consists essentiallyof:

a. from about 30 to about 37 mole percent Li₂ O;

b. from 0 to about 4 mole percent of at least one oxide selected fromthe group consisting of Cs₂ O and Rb₂ O;

c. from 0 to about 6 mole percent La₂ O₃ ;

d. from about 2 to about 8 mole percent Pr₂ O₃ ;

e. from about 2 to about 8 mole percent of at least one oxide selectedfrom the group consisting of Ta₂ O₅ and Nb₂ O₅ ;

f. from 0 to about 4 mole percent UO₂ ; and

g. the balance SiO₂ ;

with the proviso that the sum of the mole percentages of La₂ O₃ and Pr₂O₃ is equal to or less than about 12.

In another preferred embodiment, such glass composition consistsessentially of:

a. from about 30 to about 37 mole percent Li₂ O;

b. from 0 to about 4 mole percent of at least one oxide selected fromthe group consisting of Cs₂ O and Rb₂ O;

c. from about 2 to about 8 mole percent La₂ O₃ ;

d. from 0 to about 6 mole percent Ta₂ O₅ ;

e. from about 2 to about 8 mole percent Nb₂ O₅ ;

f. from 0 to about 4 mole percent UO₂ ; and

g. the balance SiO₂ ;

with the proviso that the sum of the mole percentages of Ta₂ O₅ and Nb₂O₅ is equal to or less than about 10.

In a more preferred embodiment, such glass composition consistsessentially of:

a. from about 31 to about 35 mole percent Li₂ O;

b. from about 1 to about 3 mole percent of at least one oxide selectedfrom the group consisting of Cs₂ O and Rb₂ O;

c. from 0 to about 4 mole percent La₂ O₃ ;

d. from about 3 to about 6 mole percent Pr₂ O₃ ;

e. from about 4 to about 8 mole percent of at least one oxide selectedfrom the group consisting of Ta₂ O₅ and Nb₂ O₅ ;

f. from 0 to about 2 mole percent UO₂ ; and

g. the balance SiO₂.

In another more preferred embodiment, such glass composition consistsessentially of:

a. from about 31 to about 35 mole percent Li₂ O;

b. from about 1 to about 3 mole percent of at least one oxide selectedfrom the group consisting of Cs₂ O and Rb₂ O;

c. from about 3 to about 7 mole percent La₂ O₃ ;

d. from 0 to about 4 mole percent Ta₂ O₅ ;

e. from about 2 to about 7 mole percent Nb₂ O₃ ;

f. from 0 to about 2 mole percent UO₂ ; and

g. the balance SiO₂ ;

with the proviso that the sum of the mole percentages of Ta₂ O₅ and Nb₂O₅ is equal to or less than about 10.

The present invention is further illustrated, but not limited, by theexamples which follow. In the examples, pre-melt compositions are shown.Each component is expressed in terms of the mole percentage of aspecific oxide for the sake of clarity and simplicity. It is to beunderstood, however, that the components of any given glass compositioncan be selected from any compound which will yield an oxide upon fusion.For example, lithium carbonate often is preferred over lithium oxide asa pre-melt component because of its availability and relative inertness.Thus, carbonates, hydroxides, nitrates, and other such compounds, aswell as oxides, can be used.

Each glass was prepared by mixing the pure raw materials in the drystate and melting the resulting mixture in a platinum crucible at 1500°C. for five hours, typically in an electric muffle furnace, withintermittant mixing. Other temperatures, e.g., from about 1100° to about1700° C., and heating times, e.g., from about 1 to about 7 hours, can beemployed, however, as is well known by those having ordinary skill inthe art.

Glass membranes then were prepared in accordance with well-knownprocedures. Typically, one end of a pre-heated glass tubing having theappropriate coefficient of expansion and softening point was dipped intothe molten pH electrode glass composition contained in a platinumcrucible and then withdrawn. In the case where a bulbous membrane wasdesired, the molten glass adhering to the tubing was blown to thedesired size and shape.

EXAMPLES 1-11

Pre-melt compositions for a number of glasses, exemplifying somepreferred glasses and additional glasses which illustrate the broaderlimits of the present invention, are shown in Table I. This table alsoincludes four prior art or control glasses which are given letterdesignations.

                                      TABLE I                                     __________________________________________________________________________    Pre-Melt Compositions of Various pH Glasses                                   Example                                                                       or   Composition (Mole Percent)                                               Control                                                                            Li.sub.2 O                                                                        Cs.sub.2 O                                                                       Rb.sub.2 O                                                                        BaO                                                                              La.sub.2 O.sub.3                                                                  Pr.sub.2 O.sub.3                                                                  CeO.sub.2                                                                         Ta.sub.2 O.sub.5                                                                  Nb.sub.2 O.sub.5                                                                  UO.sub.2                                                                         SiO.sub.2                           __________________________________________________________________________    1    33  2             6       6          53                                  2    33  2             6       6       2  51                                  3    33  2         3   3       6          53                                  4    33  2         6           3   3      53                                  5    33  2         6               6      53                                  6    33     2      6               6      53                                  7    29  2             2       2          65                                  8    33  2             2       8          55                                  9    37  2             8       8          45                                  10   30                6       6          58                                  11   33  2             6           6      53                                  A    33  2         6           6          53                                  B    30            6           6          58                                  C    28  2      5  2                      63                                  D      24.4     7    2.5   4.5              61.6                              __________________________________________________________________________

Control glasses A and B are typical of the compositions disclosed inU.S. Pat. No. 4,028,196. Control glasses C and D are representative ofthe compositions disclosed in U.S. Pat. Nos. 2,444,845 and 3,480,536,respectively.

Table II summarizes electrical resistance properties of the glasses ofTable I, although not all properties were determined for each glass.Other properties for these glasses are summarized in Table III.

                  TABLE II                                                        ______________________________________                                        Electrical Resistance Properties of Various pH Glasses                        Property                                                                      Example          Acid-Aged Sensors.sup.b                                                                     Base-Aged Sensors.sup.b                        or     Bulk Glass                                                                              Surface Resistivity.sup.c                                                                   Surface Resistivity.sup.c                      Control                                                                              Resistivity                                                                             0° C.                                                                           25° C.                                                                        0° C.                                                                         25° C.                         ______________________________________                                        1      9.0       1900      74     470    88                                   2      8.9       1800      68    1600   110                                   3      9.1       2100      86    2600   180                                   4      9.2       2000      67    2500   111                                   5      9.2       3100     110    2900   159                                   6      9.2       1900      64    3000   140                                   7      9.6                                                                    8      8.4        780      30    2500   160                                   9      8.4                                                                    10     9.0       2600     110    3000   260                                   11     9.0                                                                    A      9.2       3000     140    4700   120                                   B      9.2       3400     130    2600   180                                   C      10.5      6400     370                                                 D      11.3      9000     610    23000  1300                                  ______________________________________                                         .sup.a Log D.C. bulk resistivity in ohmcm, determined at about 25°     C. for untreated glass.                                                       .sup.b Data from operational electrode assemblies after accelerated aging     for 24 hours at 90° C. in 0.02 N H.sub.2 SO.sub.4 (acid) or 0.01 N     NaOH (base).                                                                  .sup.c Values in megohmscm.sup.2                                         

                                      TABLE III                                   __________________________________________________________________________    Other Properties of Various pH Glasses                                        Bulk Glass        Acid-Aged Sensors.sup.a                                                                           Base-Aged Sensors.sup.b                 Example   Chemical                                                                              Slope.sup.e         Slope.sup.e                             or   Density                                                                            Durability                                                                            (% Theoretical)                                                                        Sodium                                                                            Asymmetry.sup.g                                                                      (% Theoretical)                                                                        Sodium                                                                            Asymmetry.sup.g            Control                                                                            (g/cc)                                                                             Acid.sup.c                                                                        Base.sup.d                                                                        pH 0-9                                                                            pH 9-14                                                                            error                                                                             (MV)   pH 0-9                                                                            pH 9-14                                                                            error.sup.f                                                                       (MV)                       __________________________________________________________________________    1    3.80 0.171                                                                             0.103                                                                             100.0                                                                             100.3                                                                              30  -7     99.9                                                                              100.3                                                                              14   0                         2    3.94 0.171                                                                             0.089                                                                             100.2                                                                             98.0 28  -9     99.8                                                                               97.6                                                                              18   11                        3    3.81 0.180                                                                             0.053                                                                              99.0                                                                             99.5 50  -6     99.4                                                                              100.4                                                                              14  -9                         4    3.58 0.177                                                                             0.131                                                                              99.8                                                                             100.1                                                                              24  -7     100.2                                                                              99.5                                                                              12  -7                         5    3.37 0.181                                                                             0.163                                                                              99.8                                                                             99.0 24  -11    99.7                                                                              101.1                                                                              13  -14                        6    3.32 0.191                                                                             0.152                                                                             100.1                                                                             99.9 24  -9     99.9                                                                              100.5                                                                              14  -12                        8                 93  102  121 - 17   91  100  125 -3                         9                                                                             10                96   99  76  -18    93   98  82   1                         11                                                                            A    3.80 0.174                                                                             0.120                                                                             100.0                                                                             98.5 27  -19    99.5                                                                               99.7                                                                              11  -1                         B    3.71 0.149                                                                             0.046                                                                              99.7                                                                             99.0 56  -18    99.1                                                                               99.7                                                                              48   84                        C    2.95 0.349                                                                             0.425                                                           D    3.17 0.051                                                                             0.078                                                           __________________________________________________________________________     .sup.a Data from operational electrode assemblies after accelerated aging     for 24 hours at 90° C. in 0.02 N H.sub.2 SO.sub.4 (acid).              .sup.b Data from operational electrode assemblies after accelerated aging     for 24 hours at 90° C. in 0.01 N NaOH (base).                          .sup.c,d Powder durability tests in .sup.c 0.02 N H.sub.2 SO.sub.4 (acid)     and .sup.d 0.01 N NaOH (base). Samples were prepared according to ASTM        designation C22573 in "Annual Book of ASTM Standards", Part 17, Sections      14-16, American Society for Testing and Materials, Philadelphia, 1978;        analyses were carried out in accordance with the procedure of D. L.           Rothermel and M. E. Nordberg, Ceramic Bulletin, 31, 324 (1952), except        leachate volumes of 25 ml. were employed instead of 5 ml. Results are         expressed as weight percent total alkali removed under the specified          conditions, reported as equivalent weight percent Na.sub.2 O.                 .sup.e Measured by comparison with a standard platinum hydrogen electrode     in 1 M HCl, 1 M KOH, and 0.01 M borax buffer (pH 9.18).                       .sup.f Measured by comparison with a standard platinum hydrogen electrode     in 1 N NaOH.                                                                  .sup.g Measured in pH 7, 0.05 M phosphate buffer with an electrode filled     with a similar solution.                                                 

As the data in Tables I-III show, the present invention provides pHglasses which exhibit especially low surface resistivities and which, asa consequence, are less susceptible to polarization. The glass ofExample 1 has been found to be especially useful in the preparation ofrugged and miniaturized electrodes requiring especially low electricalresistance properties and for flat membrane electrodes requiring goodglass workability. The glasses of Examples 2-6, inclusive, and othersrelated thereto in formulation, also are similarly useful for theconstruction of rugged, miniaturized, and flat membrane electrodes. Theglasses of Examples 1-6, inclusive, have been found to perform well inconventional, bulb-type pH electrodes.

The acid-aged and base-aged data in Table II and the chemical durabilitydata in Table III indicate the effects of accelerated aging in therespective reagents at 90° C. for 24 hours.

Table II shows the electrical resistivities of the glasses of theexamples and the control glasses. Note that the glasses of Examples 1,2, and 3 are lower in bulk resistivity than all of the control glasses.The glasses of Examples 4, 5, and 6 are as low in bulk resistivity asglass A, the control glass lowest in bulk resistivity. Acid-agedelectrodes fabricated from the glasses of Examples 1, 2, 3, 4, and 6exhibit surface resistivities at both 0° C. and 25° C. which aresignificantly lower than the corresponding resistivities of the controlglass electrodes. In base-aged electrodes, the glasses of the first sixexamples exhibit surface resistivities at 0° C. which generally arelower than the corresponding resistivities of the control glasses. Thus,the glasses of the present invention typically have advantageousresistance properties, compared to the control glasses, under mostconditions of use. For example, the low surface resistance values at 0°C. of the glasses of Examples 1-6 indicate that these glasses areespecially well-suited for electrodes having low temperatureapplications.

Table III shows selected other properties which further characterize theglasses of the present invention and demonstrate the suitability of suchglasses in the construction of operational pH electrodes. The tabulatedchemical durability data indicate the total of all alkali metal oxidecomponents extracted from pulverized glass samples exposed to theindicated solutions at 90° C. for four hours; preferred values are belowabout 0.20. The asymmetry potential was measured in pH 7.0 buffersolution; a satisfactory value is 0±about 20 mV.

Our findings can be summarized as follows. In general, current pHglasses incorporate La₂ O₃ as a glass network modifier. We have found,however, that partial or complete substitution of Pr₂ O₃ for La₂ O₃results in pH glass compositions having reduced surface resistivity, aswell as reduced bulk resistivity. Similarly, we have found that theelectrical resistance properties of glasses containing both La₂ O₃ andTa₂ O₅ can be improved (i.e., lowered) by replacing part of the Ta₂ O₅by Nb₂ O₅. Frequently, the simultaneous addition of UO₂ gives additionaladvantages in workability and reduced bulk electrical resistance.Finally, Cs₂ O is incorporated in pH glass compositions to reduce sodiumerror. However, Rb₂ O can be substituted for Cs₂ O without adverseeffects, and sometimes with advantage.

Some of the properties of glasses at the claimed compositional limitsare shown by the glasses of Examples 7-11, inclusive. Glasses containingless than about 30 mole percent Li₂ O are likely to have undesirablyhigh electrical resistance properties which render such glassesunsuitable for use in electrodes having small or thick sensors. On theother hand, glasses containing greater than about 37 mole percent Li₂ Oare likely to have poor workability and durability. Compositionsexceeding 4 mole percent in Cs₂ O or Rb₂ O are likely to have excessiveelectrical resistivity properties. Glasses containing less than theminimum required amounts of La₂ O₃ and Pr₂ O₃ or Ta₂ O₅ and Nb₂ O₅ arelikely to have excessive alkali error or poorer workability, whereasglasses containing more than the maximum amounts allowed are likely tohave excessive electrical resistance properties or poor meltcharacteristics. While the performance of some of the glasses at or nearthe claimed compositional limits would be unsatisfactory for use infull-range pH electrodes, some of the adversely-affected properties suchas alkali error and limited range would not necessarily be a liabilityin less stringent applications such as the instrumental measurement ofblood pH where such glasses could be used to advantage.

Having thus disclosed the invention, many variations thereof will beapparent to those having ordinary skill in the art without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A glass composition for use in pH-responsiveglass electrodes consisting essentially of (in mole percent on the oxidebasis):a. from about 30 to about 37 mole percent Li₂ 0; b. from 0 toabout 4 mole percent of at least one oxide selected from the groupconsisting of Cs₂ O and Rb₂ O; c. from about 2 to about 12 mole percentof at least one oxide selected from the group consisting of La₂ O₃ andPr₂ O₃ ; d. from about 2 to about 10 mole percent of at least one oxideselected from the group consisting of Ta₂ O₅ and Nb₂ O₅ ; e. from 0 toabout 4 mole percent UO₂ ; and f. the balance SiO₂ ;with the provisothat there is present in the composition at least about 2 mole percentof at least one oxide selected from the group consisting of Pr₂ O₃ andNb₂ O₅.
 2. The composition of claim 1 which consists essentially of:a.from about 30 to about 37 mole percent Li₂ O; b. from 0 to about 4 molepercent of at least one oxide selected from the group consisting of Cs₂O and Rb₂ O; c. from 0 to about 6 mole percent La₂ O₃ ; d. from about 2to about 8 mole percent Pr₂ O₃ ; e. from about 2 to about 8 mole percentof at least one oxide selected from the group consisting of Ta₂ O₅ andNb₂ O₅ ; f. from 0 to about 4 mole percent UO₂ ; and g. the balance SiO₂;with the proviso that the sum of the mole percentages of La₂ O₃ and Pr₂O₃ is equal to or less than about
 12. 3. The composition of claim 2which consists essentially of:a. from about 31 to about 35 mole percentLi₂ O; b. from about 1 to about 3 mole percent of at least one oxideselected from the group consisting of Cs₂ O and Rb₂ O; c. from 0 toabout 4 mole percent La₂ O₃ ; d. from about 3 to about 6 mole percentPr₂ O₃ ; e. from about 4 to about 8 mole percent of at least one oxideselected from the group consisting of Ta₂ O₅ and Nb₂ O₅ ; f. from 0 toabout 2 mole percent UO₂ ; and g. the balance SiO₂.
 4. The compositionof claim 3 which consists essentially of 33 mole percent Li₂ O, 2 molepercent Cs₂ O, 6 mole percent Pr₂ O₃, 6 mole percent Ta₂ O₅, and 53 molepercent SiO₂.
 5. The composition of claim 3 which consists essentiallyof 33 mole percent Li₂ O, 2 mole percent Cs₂ O, 6 mole percent Pr₂ O₃, 6mole percent Ta₂ O₅, 2 mole percent UO₂, and 51 mole percent SiO₂. 6.The composition of claim 3 which consists essentially of 33 mole percentLi₂ O, 2 mole percent Cs₂ O, 3 mole percent La₂ O₃, 3 mole percent Pr₂O₃, 6 mole percent Ta₂ O₅, and 53 mole percent SiO₂.
 7. The compositionof claim 1 which consists essentially of:a. from about 30 to about 37mole percent Li₂ O; b. from 0 to about 4 mole percent of at least oneoxide selected from the group consisting of Cs₂ O and Rb₂ O; c. fromabout 2 to about 8 mole percent La₂ O₃ ; d. from 0 to about 6 molepercent Ta₂ O₅ ; e. from about 2 to about 8 mole percent Nb₂ O₅ ; f.from 0 to about 4 mole percent UO₂ ; and g. the balance SiO₂ ;with theproviso that the sum of the mole percentages of Ta₂ O₅ and Nb₂ O₅ isequal to or less than about
 10. 8. The composition of claim 7 whichconsists essentially of:a. from about 31 to about 35 mole percent Li₂ O;b. from about 1 to about 3 mole percent of at least one oxide selectedfrom the group consisting of Cs₂ O and Rb₂ O; c. from about 3 to about 7mole percent La₂ O₃ ; d. from 0 to about 4 mole percent Ta₂ O₅ ; e. fromabout 2 to about 7 mole percent Nb₂ O₅ ; f. from 0 to about 2 molepercent UO₂ ; and g. the balance SiO₂ ;with the proviso that the sum ofthe mole percentages of Ta₂ O₅ and Nb₂ O₅ is equal to or less than about10.
 9. The composition of claim 8 which consists essentially of 33 molepercent Li₂ O, 2 mole percent Cs₂ O, 6 mole percent La₂ O₃, 3 molepercent Ta₂ O₅, 3 mole percent Nb₂ O₅, and 53 mole percent SiO₂.
 10. Thecomposition of claim 8 which consists essentially of 33 mole percent Li₂O, 2 mole percent Cs₂ O, 6 mole percent La₂ O₃, 6 mole percent Nb₂ O₅and 53 mole percent SiO₂.
 11. The composition of claim 8 which consistsessentially of 33 mole percent Li₂ O, 2 mole percent Rb₂ O, 6 molepercent La₂ O₃, 6 mole percent Nb₂ O₅, and 53 mole percent SiO₂.
 12. Aglass electrode having a pH-responsive membrane of glass having acomposition consisting essentially of (in mole percent on the oxidebasis):a. from about 30 to about 37 mole percent Li₂ O; b. from 0 toabout 4 mole percent of at least one oxide selected from the groupconsisting of Cs₂ O and Rb₂ O; c. from about 2 to about 12 mole percentof at least one oxide selected from the group consisting of La₂ O₃ andPr₂ O₃ ; d. from about 2 to about 10 mole percent of at least one oxideselected from the group consisting of Ta₂ O₅ and Nb₂ O₅ ; e. from 0 toabout 4 mole percent UO₂ ; and f. the balance SiO₂ ;with the provisothat there is present in the composition at least about 2 mole percentof at least one oxide selected from the group consisting of Pr₂ O₃ andNb₂ O₅.
 13. The electrode of claim 12, in which the pH-responsivemembrane of glass has a composition consisting essentially of:a. fromabout 30 to about 37 mole percent Li₂ O; b. from 0 to about 4 molepercent of at least one oxide selected from the group consisting of Cs₂O and Rb₂ O; c. from 0 to about 6 mole percent La₂ O₃ ; d. from about 2to about 8 mole percent Pr₂ O₃ ; e. from about 2 to about 8 mole percentof at least one oxide selected from the group consisting of Ta₂ O₅ andNb₂ O₅ ; f. from 0 to about 4 mole percent UO₂ ; and g. the balance SiO₂;with the proviso that the sum of the mole percentages of La₂ O₃ and Pr₂O₃ is equal to or less than about
 12. 14. The electrode of claim 13, inwhich the pH-responsive membrane of glass has a composition consistingessentially of:a. from about 31 to about 35 mole percent Li₂ O; b. fromabout 1 to about 3 mole percent of at least one oxide selected from thegroup consisting of Cs₂ O and Rb₂ O; c. from 0 to about 4 mole percentLa₂ O₃ ; from about 3 to about 6 mole percent Pr₂ O₃ ; e. from about 4to about 8 mole percent of at least one oxide selected from the groupconsisting of Ta₂ O₅ and Nb₂ O₅ ; f. from 0 to about 2 mole percent UO₂; and g. the balance SiO₂.
 15. The electrode of claim 14, in which thepH-responsive membrane of glass has a composition consisting essentiallyof 33 mole percent Li₂ O, 2 mole percent Cs₂ O, 6 mole percent Pr₂ O₃, 6mole percent Ta₂ O₅, and 53 mole percent SiO₂.
 16. The electrode ofclaim 14, in which the pH-responsive membrane of glass has a compositionconsisting essentially of 33 mole percent Li₂ O, 2 mole percent Cs₂ O, 6mole percent Pr₂ O₃, 6 mole percent Ta₂ O₅, 2 mole percent UO₂, and 51mole percent SiO₂.
 17. The electrode of claim 14, in which thepH-responsive membrane of glass has a composition consisting essentiallyof 33 mole percent Li₂ O, 2 mole percent Cs₂ O, 3 mole percent La₂ O₃, 3mole percent Pr₂ O₃, 6 mole percent Ta₂ O₅, and 53 mole percent SiO₂.18. The electrode of claim 12, in which the pH-responsive membrane ofglass has a composition consisting essentially of:a. from about 30 toabout 37 mole percent Li₂ O; b. from 0 to about 4 mole-percent of atleast one oxide selected from the group consisting of Cs₂ O and Rb₂ O;c. from about 2 to about 8 mole percent La₂ O₃ ; d. from 0 to about 6mole percent Ta₂ O₅ ; e. from about 2 to about 8 mole percent Nb₂ O₅ ;f. from 0 to about 4 mole percent UO₂ ; and g. the balance SiO₂ ;withthe proviso that the sum of the mole percentages of Ta₂ O₅ and Nb₂ O₅ isequal to or less than about
 10. 19. The electrode of claim 18, in whichthe pH-responsive membrane of glass has a composition consistingessentially of:a. from about 31 to about 35 mole percent Li₂ O; b. fromabout 1 to about 3 mole percent of at least one oxide selected from thegroup consisting of Cs₂ O and Rb₂ O; c. from about 3 to about 7 molepercent La₂ O₃ ; d. from 0 to about 4 mole percent Ta₂ O₅ ; e. fromabout 2 to about 7 mole percent Nb₂ O₅ ; f. from 0 to about 2 molepercent UO₂ ; and g. the balance SiO₂ ;with the proviso that the sum ofthe mole percentages of Ta₂ O₅ and Nb₂ O₅ is equal to or less than about10.
 20. The electrode of claim 19, in which the pH-responsive membraneof glass has a composition consisting essentially of 33 mole percent Li₂O, 2 mole percent Cs₂ O, 6 mole percent La₂ O₃, 3 mole percent Ta₂ O₅, 3mole percent Nb₂ O₅, and 53 mole percent SiO₂.
 21. The electrode ofclaim 19, in which the pH-responsive membrane of glass has a compositionconsisting essentially of 33 mole percent Li₂ O, 2 mole percent Cs₂ O, 6mole precent La₂ O₃, 6 mole percent Nb₂ O₅, and 53 mole precent SiO₂.22. The electrode of claim 19, in which the pH-responsive membrane ofglass has a composition consisting essentially of 33 mole percent Li₂ O,2 mole percent Rb₂ O, 6 mole percent La₂ O₃, 6 mole percent Nb₂ O₅, and53 mole percent SiO₂.